An Israeli offensive against Hamas militants in the Gaza Strip is in its third day, with the Palestinian death toll from air strikes reaching at least 310.

A tally Monday by a U.N. aid agency said at least 51 civilians have been killed in Gaza since Saturday. A (UNRWA) spokesman said the estimate is based on data from medical and hospital sources in the Gaza Strip.

Palestinian officials said the raids have killed at least 180 Hamas militants.

Witnesses said today Israeli warplanes destroyed the Hamas-run Interior Ministry in Gaza City and made at least five strikes against the Islamic University in the Gaza Strip.

Israel says the offensive is in response to repeated militant attacks from Gaza against Israeli territory.

Israel's army today declared areas around the Gaza Strip a "closed military zone," a move that in the past has been followed by ground assaults throughout Gaza.

Earlier today, the mayor of the southern Israeli city Ashkelon said one Israeli was killed and eight others were wounded in a Gaza rocket attack.

Rockets fired by Hamas have been reaching farther into Israel than in the past.

On Sunday, at least one missile struck near the port city of Ashdod, some 30 kilometers north of Gaza.

Israeli Prime Minister Ehud Olmert says the operation that began Saturday may last a long time. However, Foreign Minister Tzipi Livni says Israel has no plans to re-occupy the territory, as it did for 38 years ending in 2005.

Palestinian officials say the attacks have wounded more than 900 people.

Israel allowed an aid convoy into Gaza Sunday, briefly easing its blockade on the Hamas-run territory.